The present invention relates to circuit having a bootstrap charger suitable for using in a switching mode power supply.
A bootstrap charger is a circuit that is responsible for recharging a bootstrap capacitor which forms the supply for high-side switching circuits which for instance are used in a switching mode power supply.
A conventional bootstrap charger is depicted in FIG. 4. A bootstrap capacitor C_BTS is connected between a floating terminal PH of a switching mode power supply and a supply terminal CHARGE via a diode D. When the voltage V_PH of the floating terminal PH goes down close to a reference potential GND, the bootstrap capacitor C_BTS is charged via the diode D until the voltage drop over the bootstrap capacitor C_BTS reaches a certain voltage value V_BTS. If the voltage of the floating terminal PH goes up again the diode D inhibits a discharging of the bootstrap capacitor C_BTS into the supply terminal CHARGE, such that the bootstrap capacitor C_BTS can serve as a floating bootstrap supply providing a bootstrap supply voltage V_BTS with respect to the floating terminal PH. The bootstrap capacitor C_BTS is recharged each time the voltage V_PH of the floating terminal PH goes down close to the reference potential GND. Then the bootstrap supply voltage V_BTS is equal to the supply voltage provided by the supply terminal CHARGE minus the voltage drop over the diode D.
The supply voltage of a bootstrap charger as described above has to be exactly regulated in order to provide a constant bootstrap supply voltage V_BTS. The floating bootstrap capacitor C_BTS could also be be charged via a voltage regulator, but in such an embodiment the voltage drop over the regulator would be unacceptably high, or, if a more sophisticated circuit is used to avoid this, the necessary chip area would be too high.
The voltage drop between the supply voltage and the bootstrap supply voltage is rather high, that is the voltage drop over the diode and over the voltage regulator devices. Furthermore the imprecise control of the bootstrap voltage, a long response time during switching, and the required chip area for the charger underline the need for further improvement.
For these and other reasons, there is a need for the present invention.